This application is the continuation of a proposal to determine cellular mechanisms of water and solute transport in specific segments of mammalian renal tubule. We will use the in vitro isolated tubule technique (co-developed by the P.I.), conventional and ion-selective microelectrodes, and micro-analysis of tubule enzymes and substrates in the proposed studies. We have been working on and will continue to study three general areas of cellular physiology. I. Urate and PAH transport. The renal excretion of organic anions is important for the elimination of endogenous and exogenous substances from the body. For example, abnormal urate excretion can lead to gout. We have found a substance in the serum of normal rabbits that inhibits urate secretion in rabbit proximal tubules. We will use chromatographic methods to purify the inhibitor in order to determine the cellular mechanism of urate transport inhibition. We will study urate transport in human kidney tissue to see if the inhibitor in human serum alters urate transport in human proximal tubules. We have shown that some aryl anions in plasma cause proximal tubules to secrete rather than absorb fluid under certain conditions. We will determine the mechanisms of fluid secretion driven by Pah transport in S2 segments of the proximal tubule. II. Cell volume regulation. We have shown that mammalian renal cells regulate their volume in anisotonic media. We will determine the cellular mechanisms of volume regulation in rabbit and human tubules in hypotonic media. These studies will improve the understanding of cell volume regulation in patients with severe hyponatremia. III. Vanadate. Vanadium is a trace metal found normally in renal tubule cells. The oxyanion derivative, vandate, is a powerful inhibitor of the sodium pump and is natriuretic in intact animals. We will study isolated perfused proximal and collecting tubules to determine the potential role of vanadate in the regulation of salt and water transport. Vanadium may be a key element in the regulation of renal salt and water excretion, and in the pathogenesis of hypertension in humans. As in previous applications, the proposed inquires are directed at basic cellular processes of pathogenetic significance in clinical fluid and electrolyte disorders.